MPG to L/100km Converter

Why mpg and L/100km are inverse, and the magic constant 235.21

US miles per gallon (mpg) measures how far you go per unit of fuel, so bigger is better. Metric liters per 100 kilometers (L/100km) measures how much fuel it takes to go a fixed distance, so smaller is better. They are reciprocals, which is why the conversion is a division, not a multiplication:

L/100km = 235.214583 ÷ US mpg

Where does 235.21 come from? You travel 100 km, convert to miles (100 ÷ 1.609344 = 62.1371 mi), then convert US gallons to liters (1 gal = 3.785411784 L). The constant is 100 ÷ 1.609344 × 3.785411784 = 235.214583.

Worked example: a car rated 30 mpg uses 235.214583 ÷ 30 = 7.8405 L/100km. A pickup at 18 mpg uses 235.214583 ÷ 18 = 13.067 L/100km. Because the relationship is inverse, a 2 mpg gain from 18 to 20 saves far more fuel than the same 2 mpg gain from 48 to 50.

US vs imperial (UK) gallons, the mistake that ruins comparisons

The single biggest trap in fuel-economy conversion is the gallon. The US gallon is 3.785411784 L; the imperial (UK) gallon is 4.54609 L, about 20% bigger. So "mpg" means two different things depending on the country:

• US mpg → L/100km: divide 235.214583 by mpg.
• UK (imperial) mpg → L/100km: divide 282.480936 by mpg.

Because the UK gallon is larger, the same real-world car shows a higher mpg number in the UK than in the US. A car that gets 30 US mpg is about 36 UK mpg, a 20% difference that is purely a unit artifact, not a different car. This converter uses US gallons, matching what Americans see on a window sticker and on the US EPA's fueleconomy.gov. The exact US-gallon volume of 3.785411784 L is fixed by NIST Special Publication 811. If you are reading a British magazine, divide by 282.481 instead. Never mix the two in the same comparison.

MPG to L/100km reference table

Every row is 235.214583 ÷ US mpg. Notice how the L/100km curve flattens at high mpg and explodes at low mpg.

Numeric check: 25 mpg = 9.4086 L/100km (235.214583 ÷ 25) and 50 mpg = 4.7043 L/100km (235.214583 ÷ 50). The jump from 10 mpg to 15 mpg cuts consumption from 23.52 to 15.68 L/100km, a 7.8 L/100km saving, while going from 45 to 50 mpg saves less than 0.6 L/100km. That asymmetry is the whole point of using L/100km.

Read the table with the inverse rule in mind: halving the mpg doubles the L/100km, and doubling the mpg halves it. So 20 mpg (11.76) is exactly twice the consumption of 40 mpg (5.88), which is why small mpg gains at the low end matter so much.

Why the inverse relationship matters for fuel savings

When you shop in mpg you instinctively think in equal steps: "+5 mpg is +5 mpg." But fuel burned is what you pay for, and that scales with L/100km, which is inverse. The practical lesson:

• Trading a 15 mpg truck for a 20 mpg truck (a 5 mpg gain) cuts fuel use by about 25%.
• Trading a 40 mpg car for a 45 mpg car (also 5 mpg) cuts fuel use by only about 11%.

Over 15,000 miles a year at $3.50 per gallon: the 15→20 mpg swap saves roughly 175 gallons (~$612), while the 40→45 mpg swap saves about 42 gallons (~$146). Same "+5 mpg" on paper, four times the real saving. European regulators and the EPA's own consumer guidance both use L/100km-style thinking for exactly this reason. This is also why the US EPA prints annual fuel cost on the window sticker rather than just mpg.

Converting the other direction and common mistakes

The conversion is symmetric, the same constant works both ways, because the two quantities are reciprocals:

• US mpg = 235.214583 ÷ L/100km
• L/100km = 235.214583 ÷ US mpg

So 7.8405 L/100km → 235.214583 ÷ 7.8405 = 30 mpg, back where we started. Frequent mistakes:

• Using the UK constant with US gallons. Divide by 282.481 only for imperial mpg.
• Assuming higher L/100km is better. It is worse, more fuel to go the same distance.
• Comparing US and UK mpg directly. The same car reads about 20% higher in UK mpg.
• Forgetting that the relationship is nonlinear. A 1 mpg change is worth different amounts at 15 mpg versus 45 mpg.

The US Environmental Protection Agency publishes official ratings in mpg on fueleconomy.gov; most of the rest of the world publishes L/100km. This converter keeps you honest across both.

One more check worth doing: because the two are reciprocals, a car that uses 10 L/100km is 235.214583 ÷ 10 = 23.5 US mpg, and a car that uses 5 L/100km is 47.0 US mpg. The whole 5-to-15 L/100km band maps to roughly 47 down to 16 US mpg.

Comparing US mpg, UK mpg and MPGe

Three different "economy" numbers get compared as if they were the same thing, and they are not:

• US mpg, miles per US gallon (3.785 L). What Americans see on fueleconomy.gov.
• UK (imperial) mpg, miles per imperial gallon (4.546 L), about 20% bigger, so the number is about 20% higher for the same car.
• MPGe, miles per gallon-equivalent, used for EVs and plug-in hybrids. It converts electrical energy to the heat content of a gallon of gasoline (33.7 kWh per US gallon) so electric and gas cars can be listed side by side.

Worked example: a car rated 30 US mpg is about 36 UK mpg (30 × 1.20095) and uses 7.8405 L/100km. The same car rated 50 UK mpg is only 41.6 US mpg (50 × 0.83267), which is 235.214583 ÷ 41.6 = 5.65 L/100km, one car, three numbers. To move between the two mpg scales, multiply US mpg by 1.20095 to get UK mpg, or multiply UK mpg by 0.83267 to get US mpg. For L/100km, always divide 235.214583 by the US figure. Mixing a UK mpg value into the US formula understates consumption by about 20%, enough to make a car look far thriftier than it is.

Quick mental conversions for fuel economy

When you are reading a spec sheet abroad or comparing a rental car, two mental tricks help:

• mpg to L/100km: divide 235 by the US mpg. A 20 mpg truck is 235 ÷ 20 = 11.8 L/100km (exact 11.76).
• L/100km to mpg: divide 235 by the L/100km value. An 8 L/100km car is 235 ÷ 8 = 29.4 US mpg (exact 29.40).

Useful anchors to memorize: 30 mpg ≈ 7.8 L/100km, 25 mpg ≈ 9.4, 40 mpg ≈ 5.9, and 50 mpg ≈ 4.7. Worked example: a car advertised at 6.5 L/100km is 235 ÷ 6.5 = 36.2 US mpg, which sounds impressive until you realize it is a European figure that would read about 43 UK mpg, the same car. For anything you will quote in public, use 235.214583, not 235, because the rounding compounds across a fleet and a year of fuel receipts.

If you prefer a single number to carry in your head, remember that 235 ÷ mpg = L/100km and that a good modern car sits near 7 L/100km, or about 34 US mpg. Anything under 6 L/100km is excellent; anything over 12 is a truck or a thirsty SUV.